1. Field of the Invention
The invention relates generally to physiological test devices and associated test methods and, more particularly, to test devices and methods that utilize near infrared spectrometry and ultrasound in combination.
2. Relevant Background
Subnormal blood oxygenation arises in many medical conditions.
Fetal hypoxemia is the subnormal oxygenation of arterial blood, an example of a low oxygenation condition, and results from a variety of disorders. Korst LM, Phelan JP, et al., xe2x80x9cAcute fetal asphyxia and permanent brain injury: a retrospective analysis of current indicatorsxe2x80x9d, in J. Matern-Fetal Med. 1999; 8:275-288, studied 47 brain-injured infants and determined that fetal hypoxemia resulted from causes as follows: 14 (30%)-uterine rupture; 5 (11 %)-shoulder dystocia; 5 (11%) cord prolapse; 3 (6%)-maternal cardiac arrest; 2 (4%)-placental abruption; 1 (2%)-fetal exsanguinations; and 17 (36%)-unknown. Fetal hypoxemia may cause infant mortality, fetal death, low birth weight, and severe mental retardation.
As oxygen levels begin to decline, the fetus can respond using one or more of several compensatory mechanisms to maintain an intracellular steady state. An increase in fetal heart rate can slightly increase cardiac output, as described by Thomburg KL, Morton MJ, xe2x80x9cDevelopment of the cardiovascular systemxe2x80x9d, in: Textbook of Fetal Physiology (Thomburn G, Harding R, eds.), 1994, pp. 118-120, New York: Oxford University Press.
Where blood supply is reduced, the fetal circulatory system shunts the blood supply to vital organs such as the heart, brain, and adrenal glands to supply sufficient oxygen, as described by Peeters LL, Sheldon RD, Jones MD, et al., xe2x80x9cBlood flow to fetal organs as a function of arterial oxygen contentxe2x80x9d in Am J Obstet Gynecol 1979;135:637-645.
In case of hypoxemia, the fetus enters a state of anaerobic metabolism. Anaerobic metabolism produces 18 times less energy than is produced under aerobic conditions, negatively impacting the fetus. (Nordstrom L, and Arulkumaran S, xe2x80x9cIntrapartum Fetal Hypoxia and Biochemical Markers: A Reviewxe2x80x9d in Obstetrical and Gynecological Survey, 1998;53:10, pp.645-657). Neuronal loss due to hypoxemia occurs in two phases. A primary loss takes place at the time of the hypoxic event, and a secondary loss occurs during a reoxygenation/reperfusion phase, hours to days after the event. (Pulsinelli W, Brierly J, and Plum F, xe2x80x9cTemporal profile of neuronal damage in a model of transient forebrain ischemiaxe2x80x9d in Ann Neurol 1982; 11:491-498). The primary damage is due to deterioration of the cellular steady state including acidosis, disrupted ion distributions, and altered tissue perfusion. The damaging mechanisms of the secondary phase are metabolic changes, neurotoxicity, and circulatory changes.
Hypoxia occurs when the oxygen supply to the brain is inadequate for normal cellular function. Hypoxia can result in brain damage and/or death of the fetus. In-utero fetal hypoxia can result if the umbilical cord wraps around a fetus"" neck, thereby restricting blood flow to the head. A high-risk fetus typically lacks cerebral blood pressure regulation mechanisms that are found in adults and normal fetuses. Contractions can cause cerebral hemorrhage that lead to hypoxia in a high-risk fetus.
Technological advances such as electronic fetal monitoring and fetal oximetry have failed to reduce fetal morbidity and mortality rates. Electronic fetal monitors (EFM) use acoustic energy to record the fetal heart rate. Cardiac accelerations and decelerations on the recording are visually analyzed to determine whether the fetus is in a distress condition. EFM was introduced into labor and delivery practice decades ago and has gained widespread use despite limited effectiveness. Studies of the efficacy of EFM have produced mixed results, concluding in many cases a poor correlation with fetal outcome.
A more recent technological advance is the development of fetal oximetry. Fetal oximeters use an optical source and photodetector attached directly to the fetal head. While fetal oximetry may assist an obstetrician in assessing fetal status, usage of oximeter technology requires rupture of maternal membranes, limiting oximetry usage to later stages of labor.
A diagnostic apparatus includes a near infrared spectrophotometer and an ultrasound transducer that operate in combination to improve diagnostic measurements.
In one aspect, the need for a method to precisely position the NIRS optical sample volume in tissue is met by ultrasound guided near infrared spectrometry.
In another aspect, a catheter includes optical sources, a photodetector, ultrasound transducer array, and stabilization balloon. The display shows an optical sample volume through tissue of interest that is superimposed over the ultrasound image, and an oxygenation indicator.
In another aspect, the diagnostic apparatus includes a near infrared spectrophotometer that measures tissue oxygenation in an optical sample volume and an ultrasound imager to accurately position the optical sample volume in biological tissue or vessels.
In one embodiment, the diagnostic apparatus includes an optical source, a linear array of ultrasound transducers, and an optical photodetector arranged in the same plane so that the ultrasound sample volume interrogated by the ultrasound transducers intersects the optical sample volume formed by the optical source and detector.
In some embodiments, the optical source and photodetector are attached to rotary and linear sensors that supply position data, for example to processing element such as a computer, processor, controller, logic element, or the like. The processing element can calculate the distance between the optical source and the photodetector. The source-detector distance and tissue optical properties based on near infrared spectrophotometer measurements determine the position of the optical sample volume in the tissue. Calibration fixtures or phantoms that have optical properties similar to the tissue of interest can be used to determine the shape of the optical sample volume for a particular source-detector distance.
In another aspect, some systems may include an electronic graphics display and a processing or control element that superimposes an experimentally-determined optical sample volume over an ultrasound image on the display. The combined optical sample volume and ultrasound image display enables a clinician to use the electronic display to accurately position the optical sample volume through a desired tissue portion.
In operational aspects, the diagnostic apparatus is capable of performing a noninvasive method of precisely positioning a optical sample volume projected and received by a near infrared spectrophotometer (NIRS) in deep tissue through usage of an ultrasound imager. An example of the positioning method includes several actions such as arranging an optical source, a linear array of ultrasound transducers, and an optical photodetector in the one plane so that the ultrasound sample volume intersects the optical sample volume. An outline of the theoretical optical sample volume is superimposed over the ultrasound image on an electronic graphics display. The superimposed image on the electronic graphics display enables or facilitates the capability of a clinician to accurately position the optical sample volume through the desired tissue.
Various embodiments and examples of the diagnostic apparatus can be used for different tasks. A tissue analysis device, for example capable of taking measurements through the skin, can be used for functions such as noninvasive detection of fetal hypoxemia. A catheter device can be used for functions such as noninvasive determination of oxygenation level in tissue through vessels and body openings including but not limited to oral, rectal, nasal, and otic openings. For example a catheter may be used to establish whether oxygenation is sufficient for the heart to be successfully resuscitated by defibrillation.